I have a 1986 Chevy k10 with a rebuilt th350 tranny. It was my main vehicle for two years and I got another vehicle to go to college in so the 86 has sat for 2 years. I get in to drive it and it has a hard time shifting from 1st -2nd and from 2nd -3rd but shifts from 3rd -4th just fine. When it shifts from 1st -2nd it shifts hard around 35 MPH and has a stutter shift from 2nd -3rd. Can anyone help me diagnose the issue? Thanks in Advance!

It is knows as a one year deal called a th350c. I have one in my chevelle right now. It was a 89 year thing and was used in everything from corvette to pickups. It is not wery strong and dont take much abuse. But it is the only trans that rumor says can be run without the lockup hooked up. I have driven mine 3 years now without the lockup and it works fine. Just like a normal 350. I would start with the filter and fluid, change it and make sure its the correct level. If it dosent work look at the vacum hose to the trans modulator. If fine it could be the orings inside thet is dried out or the govenor that sticks. That means pulling the trans.

When you have the pan off. Check the s-link, that connects the external shift shaft to the internal shift valve on left side of valve body. To do so remove link, put shifter in neutral, then manually put trans in neutral and check s-link. It may need a slight tweet to go in position without moving either item. Had a Trans years ago kick my azz and a local Trans guy told me of this. Easy to overlook and as I understand is the result of reverse drops and harsh use in general. Car had similar shift issues you describe. S-link is about mid way back on left side of Trans and easily accessible with pan off. Also, check govner on right side of trans. May need to lower trans to miss exhaust. Remove round cover on right side of trans just above pan and check for free movement, may be sticking and shifting under hi govner pressure. As mentioned above also check vacuum modulator valve for fluid, diaphram may be damaged. Good lluck